The invention relates to a rolling bearing, in particular a rolling bearing suitable for engine auxiliary machines (alternator, electromagnetic clutch or intermediate pulley), and also relates to a method for manufacturing the same.
Conventionally, SUJ2 (high carbon chromium bearing steelxe2x80x942nd category) has mainly been employed as materials of bearing rings and rolling elements of rolling bearings.
Recently, in company with making automobiles small sized and light weight, auxiliary machines of engines have also been demanded to provide high performance and high output together with making the small size and the light weight. For example, bearings for alternators are subjected to high vibration or high load (around 4 G to 20 G at the acceleration of gravity) through belts accompanying rotation of high speed simultaneously with working of the engine. Therefore, in the conventional bearing for the alternator, a problem has been present that exfoliation (so called as a premature peeling) is easy to occur at an premature period of service especially in the bearing face of the outer ring being a stationary ring.
There have been proposed many techniques for preventing the premature peeling/exfoliation so as to lengthen the life of the bearing to be served under the high vibration and high load.
For example, Japanese Patent Unexamined Publication No. Hei.4-28845 proposes the use of a steel containing low carbon (C) (0.65 to 0.90%) and high chromium (Cr) (2.0 to 5.0%) in comparison with SUJ2 which has hitherto been used. This publication discusses that the premature exfoliation is caused by generation of a white structure in steel which occurs by cohesion of carbon. Thus, by employing the above mentioned steel, the carbon is prevented from diffusion to be a cause of cohesion for attaining the long life.
Japanese Patent Unexamined Publication No. Sho.62-218542 proposes the technology that a blank material is made by adding Al or Si to SUJ2 or a steel following it to be 1 to 2% therein, a bearing ring is formed with this blank material, followed by an ordinary quenching and a tempering at high temperature, thereby decreasing a residual austenite to be 8 vol % or lower. According to this art, a softening resistance by tempering is heightened by adding Al or Si. By decreasing the residual austenite, it is possible to reduce a plastic deformation in the bearing face under the high vibration and high load. Thereby, the hardness in the bearing ring is maintained high to prevent the premature exfoliation for attaining the long life.
Further, Japanese Patent Examined Publication No. Hei.7-72565 discloses that the stationary ring formed with SUJ2 is carried out with the ordinary quenching and then a subzero treatment, followed by the high temperature tempering, for reducing the residual austenite to be 10 vol % or lower in the stationary ring. That is, by reducing the residual austenite in the stationary ring, the hardness in the bearing ring is maintained high to lower the plastic deformation in the bearing face of the stationary ring under the high vibration and high load to reduce the premature exfoliation.
However, the circumstances of using rolling bearing for engine auxiliary machines have recently been severer. Therefore, a problem has arisen that the technology set forth in JP-A-4-28845 could not sufficiently check the appearance of the white structure.
A further problem is that since the arts of Japanese Patent Examined Publication No. Hei.7-72565 and Japanese Patent Unexamined Publication No. Sho.62-218542 reduce the residual austenite by the tempering of high temperature, the hardness over the entire bearing ring is lowered accordingly. If the blank material is a steel of high Al or Si as 1 to 2%, oxides based inclusions are easily formed and a rolling fatigue life is probably shortened.
The invention has been realized, paying attentions to the problems of the prior arts, and accordingly it is an object of the invention to provide a rolling bearing served under the conditions of high vibration and high load, in which the premature exfoliation is prevented and a long life of the bearing is possible, and also to provide a method for manufacturing the same.
For accomplishing the above mentioned problems, the rolling bearing of the invention is characterized in that at least the stationary ring (the bearing ring of an inner ring and an outer ring, which is at a stationary side when using) is fabricated with a steel material containing as alloying elements carbon (C) 0.80 to 1.10 wt %, silicon (Si) 0.20 to 0.50 wt %, manganese (Mn) 0.2 to 1.0 wt %, and further containing any two kinds or more of chromium (Cr), molybdenum (Mo) and vanadium (V) at rates of Cr: 1.2 to 3.5 wt %, Mo: 0.5 to 1.5 wt % and V: 0.2 to 1.0 wt %, the containing rates of Cr, Mo and V satisfying the under mentioned formula (1), subsequently subjecting to the quenching and the tempering.
1.8xe2x89xa6[Cr]xc3x970.6+[Mo]xc3x970.5+[V]xe2x89xa63.0xe2x80x83xe2x80x83(1) 
(in the formula, [Cr] shows the containing rate (wt %) of Cr in the steel material, [Mo] shows the containing rate (wt %) of Mo in the steel material, and [V] shows the containing rate (wt %) of V in the steel material).
The above-mentioned object can also be achieved by a rolling bearing according tot he present invention comprising:
a stationary ring;
a rotating ring; and
a plurality of rolling elements disposed between the stationary ring and the rotating ring,
wherein the stationary ring is made of a steel material containing,
0.80 to 1.10% by weight of C,
0.20 to 0.50% by weight of Si,
0.20 to 1.0% by weight of Mn,
and further containing any two kinds or more of
1.2 to 3.5% by weight of Cr,
0.5 to 1.5% by weight of Mo, and
0.2 to 1.0% by weight of V wt %, and wherein containing rates of Cr, Mo and V satisfying a following formula (1),
1.8xe2x89xa6[Cr]xc3x970.6+[Mo]xc3x970.5+[V]xe2x89xa63.0xe2x80x83xe2x80x83(1) 
where, [Cr] defines the containing rate (wt %) of Cr in the steel material, [Mo] indicates the containing rate (wt %) of Mo in the steel material, and [V] denotes the containing rate (wt %) of V in the steel material,
wherein the steel material is subsequently subjected to quenching and tempering.
In addition to this, the above-mentioned object can further be achieved by a method for manufacturing a component of rolling bearing comprising a stationary ring, a rotating ring and a plurality of rolling elements disposed between the stationary ring and the rotating ring, the method according to the present invention comprising:
forming a blank stationary ring with a steel material containing, as alloying elements
0.80 to 1.10% by weight of C,
0.20 to 0.50% by weight of Si, and
0.20 to 1.0% by weight of Mn,
and further containing any two kinds or more of
1.2 to 3.5% by weight of Cr,
0.5 to 1.5% by weight of Mo, and
0.2 to 1.0% by weight of V wt %, and wherein containing rates of Cr, Mo and V satisfying a following formula (1),
1.8xe2x89xa6[Cr]xc3x970.6+[Mo]xc3x970.5+[V]xe2x89xa63.0xe2x80x83xe2x80x83(1) 
where, [Cr] defines the containing rate (wt %) of Cr in the steel material, [Mo] indicates the containing rate (wt %) of Mo in the steel material, and [V] denotes the containing rate (wt %) of V in the steel material;
quenching thus formed blank stationary ring;
tempering thus quenched blank stationary ring, to thereby provide the stationary ring.
In the rolling bearing of the invention, as the steel material for fabricating at least the stationary ring, the material contains any two kinds or more of Cr, Mo and V at the above mentioned rates such that the above formula (1) is satisfied. Thereby, lots of carbides different in kinds and sizes are diffused and precipitated in the steel after the heat treatment (quenching and tempering). These carbides check the diffusion of carbon, so that the appearance of the white structure is prevented or delayed. Thus, an effect of avoiding the premature exfoliation is heightened, and the life of the bearing used under the high vibration and high load is lengthened.
Specifically, if the steel material to be used contains any two kinds or more of Cr, Mo and V, carbides such as M2C, M3C, M7C3 or M23C6 are diffused and precipitated in the steel after the heat treatment. In the V addition, fine VC and V3C are precipitated, in the Mo addition, spherical Mo2C or bar like Mo23C6 are precipitated, and in the Cr addition, cementite typed Cr3C is precipitated.
These carbides check the diffusion of carbon and have high effects to make difficult to cause dislocation, and therefore their inclusions may lengthen the life of the bearing. Among Cr, Mo and V, the V addition generating fine carbides has the highest effect, next is Cr, and Mo follows.
Herein, if [Cr]xc3x970.6+[Mo]xc3x970.5+[V] of the (1) formula (this is provisionally called as xe2x80x9cxcex1 valuexe2x80x9d) is determined to be 1.8 or higher to 3.0 or lower, it is found that the existing rates in steel of plural kinds of precipitated carbides are reasonable values for providing effects checking the carbon diffusion.
If the xcex1 value is less than 1.8, the effect checking the carbon diffusion is not fully displayed. If the xcex1 value is more than 3.0, macro-carbides lowering the bearing life is easily formed. In addition, there is much carbon forming carbides by combining Cr, Mo and V, so that carbon amount made solid in a matrix is short, and it is difficult to provide an enough hardness of the matrix.
In the rolling bearing of the invention, it is desirable that the steel material further contains titanium (Ti) and/or copper (Cu) at the rates of Ti: 0.05 to 0.20 wt % and Cu: 0.2 to 2.0 wt %.
If the steel material contains Ti and/or Cu at the rates of Ti: 0.05 to 0.20 wt % and Cu: 0.2 to 2.0 wt %, there appears precipitation of Fe3C including TiC precipitation and/or Cu grains in the steel after the heating treatment. These TiC and Cu grains are effective to check the carbon diffusion.
Critical significance for limiting the numerical value as to the containing rate of each element is as follows.
[C: 0.80 to 1.10 wt %]
C is an element made solid in the matrix to impart the hardness to steel, and combining carbide formers such as Cr, Mo and V to form carbides. If the C containing amount is less than 0.80 wt %, the carbon amount made solid in the matrix is short, the hardness demanded to the rolling bearing (57 or more in the Rockwell hardness (HRC) in a case of a scale C) might not be secured. If exceeding 1.10 wt %, macro-carbides are easily generated during steel making, and the fatigue life or the impact resistance are probably decreased.
[Si: 0.20 to 0.50 wt %]
Si is an element acting as a deoxidizer during steel making, and improving a quenching property as well as strengthening martensite of the matrix, and effective for lengthening the bearing life. If the Si containing amount is less than 0.20 wt %, these effects cannot be enough obtained. If exceeding 0.50wt %, machinability, forging property and cold working property are remarkably decreased.
[Mn: 0.2 to 1.0 wt %]
Mn is an element strengthening ferrite in steel and increasing the quenching property. If the Mn containing amount is less than 0.20 wt %, these effects are insufficient. If exceeding 1.0 wt %, an amount of the residual austenite after quenching is much to lower the hardness and the cold working property.
[Cr: 1.2 to 3.5 wt %]
Cr is an element exhibiting improvements of the quenching property, of the softening resistance by quenching and of the abrasion resistance. Further this is an element forming carbides to exhibit the above mentioned effects. If the Cr containing amount is less than 1.2 wt %, these effects cannot be substantially provided. If exceeding 3.5 wt %, not only these effects are saturated, but also macro-carbides are generated and the machinability is deteriorated.
[V: 0.2 to 1.0 wt %]
V is an element forming fine carbides and effective to increase the abrasion resistance. Further, this is an element forming carbides to exhibit the above mentioned effects. If the V containing amount is less than 0.2 wt %, these effects cannot be substantially provided. If exceeding 1.0 wt %, not only these effects are saturated, but also macro-carbides are generated and cost-up of materials arises.
[Ti: 0.05 to 0.20 wt %]
Ti is an element forming carbides and exhibiting the effect to check the carbon diffusion. If the Ti containing amount is less than 0.05 wt %, this effect cannot substantially be provided. If exceeding 0.20 wt %, this effect is saturated.
[Cu: 0.2 to 2.0 wt %]
Cu improves a corrosion resistance of steel, and precipitates as Cu grains as mentioned above. The Cu grain has the effect checking the carbon diffusion as having referred to. If the Cu containing amount is less than 0.2 wt %, this effect cannot substantiallybe provided. If exceeding 2.0 wt %, not only this effect is saturated, but also the hot working property is worsened.
If the Cu containing amount is much, flaws probably appear in the steel surface during hot-working. For avoiding this, it is preferable to add nickel (Ni) 2.0 wt % or lower (the containing amount in the whole of steel) in response to the Cu containing amount.